1. Field of the Invention
The present invention relates to a liquid crystal display device where a ferroelectric liquid crystal is employed.
2. Description of the Related Art
As a liquid crystal display device employed with a nematic liquid crystal, a typical prior art includes a twisted nematic (TN) type liquid crystal display device, and a supertwisted birefringence effect (SBE) type liquid crystal display device.
However, such a twisted nematic type liquid crystal display device has a drawback that a sufficient contrast cannot be achieved because a drive margin has become narrower as a driving mechanism is multiplexed. The supertwisted birefringence effect type liquid crystal display device with a large twisted angle, which is an improved TN type liquid crystal display device, has also drawbacks that a great capacity of display causes a reduction of a contrast and a reduction of response velocity.
Then, as an improvement over such conventional liquid crystal devices employed with a nematic liquid crystal, N. A. Clark and Lagerwall proposed in 1980 a liquid crystal display device employed with a chiral smectic-C liquid crystal, or a ferroelectric liquid crystal (see Japenese Unexamined Patent Publication 56-107216 and U.S. Pat. No. 4,367,924).
The liquid crystal display device is that which utilizes a rotation force for matching the spontaneous polarization of the ferroelectric liquid crystal for a polarity of the electric field, unlike the above-mentioned liquid crystal display device which utilizes dielectric anisotropy of liquid crystal molecules to use a electric field effect. Features of this type of liquid crystal device include bistable, memory storage, quick response, and so forth. Specifically, when the ferroelectric liquid crystal is injected into a cell with thinner cell gap, an interface affects the ferroelectric liquid crystal to untie its spiral structure, and eventually, the ferroelectric liquid crystal assumes bistable because both areas where liquid crystal molecules tilt by an angle .theta. related to a smectic layer normal line to be stable and areas where they tilt by -.theta. in the reverse direction to be stable exist mingled with one another. Applying voltage to the ferroelectric liquid crystal within the cell, the liquid crystal molecules and their respective direction of the spontaneous polarization can be completely uniform, and thus, switching a polarity of the applied voltage allows an orientation of the liquid crystal molecule to switch from one state to another, in other words, a switching operation can be provided.
With such a switching operation, birefringent light varies in the ferroelectric liquid crystal within the cell, and therefore, the cell sandwiched between two polarizers can control a light transmission. Even if the voltage application is interrupted, the orientation of the liquid crystal molecules is retained in state of voltage before the interruption of voltage application because of an orientation adjusting force of the interface, so that a memory effect can also be attained. As to a time required for the switching operation, since the spontaneous polarization of the liquid crystal and the electric field directly interact with each other, a quick response of a required time of 1/1000 or less can be obtained compared with the TN type liquid crystal display device, thereby the device becomes available for a quick display.
Then, by utilizing the feature of the ferroelectric liquid crystal such as a memory effect and quick response, attempts have been made to construct a high resolution liquid crystal display device with a large number of scanning lines in accordance with a multiplex driving system.
However, there were several disadvantages in the Clark-Lagerwall type liquid crystal display device. In its first model, it was presumed that the smectic-C phase has a layer structure named "bookshelf type" where layers lie vertical on a substrate as shown in FIG. 9. However, when a cell is created in accordance with a conventional orientation method such as rubbing and the like, the resultant switching phenomenon and optical characteristic are deviated from a prospective view a great deal, and it is observed that the practical switching operation is completely different from that in the proposed model.
As one of factors, the layer structure was analyzed using an X-ray small angle scattering method and it is found that the layer structure is that named "chevron" and having a dogleg shape [see Phys. Rev. Lett., 59, p. 2658 (1987) by T. P. Rieker, N. A. Clark et al.]. Another point different from the first model is reported that the cell assumes a twisted orientation where molecules are twisted in upper and lower substrates as well as a uniform orientation where the direction of the spontaneous polarization and liquid crystal molecules are oriented in a uniform direction [J. Phys. (France), 45, p. 143 (1984) by M. Glogarova and J. pavel].
Especially, a ferroelectric liquid crystal element having its orientation defined by rubbing has a strong adjusting force in its interface, and therefore, it is found that the element has a twisted orientation. With such an orientation, in general, no effective difference in optical molecule axis in switching between two states cannot be observed, and it is found that no high contrast characteristic can obtained. To eliminate such drawbacks, there have been presented several systems to attain a layer structure model first proposed by Clark et al., and one of them is reported that the model is created by SiO slanting vacuum evaporation, where with addition of a relatively high pre-tilt to a substrate interface, a layer structure having a tilt is attained without bends of layers.
As a second method, technology is proposed in which a layer structure is changed into a bookshelf structure by applying an alternating electric field of greater voltage to a cell with a bending structure [12th Liquid Crystal Discussion (in Nagoya) 1F16 (1986) by Satoh et al.], and according to the reports, both of the above two methods attained a high contrast characteristic. However, the above-mentioned slanting vacuum evaporation has a great problem in a stage of production that a technology of keeping a uniform deposition angle is not so simple and that the procedure includes a process performed under near vacuum. Moreover, as to a method of applying an electric field, it is difficult to uniformly change the layer structure, and many of preparation cells change back to chevron structure with time, and in other words, the method still stays in an experimental stage.
Accordingly, it is an object of the present invention to provide a liquid crystal display device of high contrast property despite its chevron structure.